# Global Illumination CMSC 435/634. Global Illumination Local Illumination – light – surface – eye – Throw everything else into ambient Global Illumination

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• Slide 1
• Global Illumination CMSC 435/634
• Slide 2
• Global Illumination Local Illumination light surface eye Throw everything else into ambient Global Illumination light surface surface eye Multiple bounces All photon paths: Reflection, refraction, diffuse Participating media
• Slide 3
• Global Illumination ambient no ambient global illumination
• Slide 4
• Radiometric Units TermSymbolUnits Radiant EnergyQJ Radiant Flux (Power) = dQ/dt W = J/s Radiant Intensity I = d /d W/sr Radiosity (exiting) B = d /dA W/m 2 Irradiance (entering) E = d /dA W/m 2 Radiance L = d 2 /(d dA) W/(sr m 2 )
• Slide 5
• Radiant Energy (Q) Total energy (Joules) Over all time, directions, area,
• Slide 6
• Radiant Flux ( ) = dQ/dt in Watts = J/s Radiant energy per unit time This is the one you probably want Unless you are measuring total energy absorbed E.g. by a plant over hours of daylight
• Slide 7
• Radiant Intensity (I) I = d /d in W/sr Radiant Flux emitted per unit solid angle Light from a point in a small cone of directions
• Slide 8
• Radiosity (B) B = d /dA in W/m 2 All light leaving a patch of surface Emitted or reflected All directions Measured per unit area
• Slide 9
• Irradiance (E) E = d /dA in W/m 2 All light entering a patch of surface All directions Measured per unit area
• Slide 10
• Radiance (L) L = d 2 /(d dA) in W/(sr m 2 ) Light entering patch of surface from a direction Per unit area Per unit solid angle Think of light coming into a patch of surface from a small cone of directions Compare to Irradiance (over all directions)
• Slide 11
• Photometric Units Considers human response How bright it seems TermSymbolUnitsName Luminous EnergyQTTalbot Luminous Flux = dQ/dt lm = T/sLumen Luminous Intensity I = d /d cd = lm/srCandella Illuminance E = d /dA lx = lm/m 2 Lux Luminance L = d 2 /(d dA) cd/m 2 Lambert Lamberts = 1 cd/cm 2
• Slide 12
• Backward Algorithms: Ray / Path Tracing Follow photons backwards: eye to light Traditional ray tracing Follow primary reflection Path tracing Monte-carlo integration Probabalistically choose path direction Many rays per pixel Kajiya 1986
• Slide 13
• Forward Algorithms: Photon Map Follow photons forward: light to eye Photon Map Bounce photons from surface to surface Collect in spatial data structure Final gather per pixel Wann Jensen and Christensen 1998
• Slide 14
• Forward Algorithms: Radiosity Diffuse only: Progressive Radiosity Lights emit Other surfaces collect rendering hemicube Then emit Cohen et al. 1988
• Slide 15
• Forward Algorithms: Radiosity Full Radiosity Form Factor = contrib of patch i on patch j Radiosity i = Emission i + FormFactor i,j * Radiosity j Solve (big) matrix form
• Slide 16
• Forward Algorithms: Virtual Point Lights (Instant Radiosity) Bounce photons Leave virtual point light at each bounce Watch out for weak singularity Light too bright near point Hayward
• Slide 17
• Bidirectional Path Tracing Trace both light and view paths Connect view path to light path Instead of view path to light Metropolis Find paths that work Mutate them to make more
• Slide 18
• 18 Bidirectional Path Tracing & Metropolis Light Transport
• Slide 19
• Interactive Rendering Viewpoint independent Diffuse surfaces only Pre-compute and store radiosity As patch/vertex colors As texture Separate solution for each light Linear combination to change lights
• Slide 20
• Interactive Rendering Viewpoint dependent Compute light probes at limited points Store in a form with direction Cube Map per probe Spherical Harmonics Precomputed Radiance Transfer Directional representation per vertex or texel

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